The present invention relates to a process for the thermal treatment of high strength aluminum alloys of the 7000 series of the Al-Zn-Mg-Cu type, containing between 0.05% and 5%, by weight, of Cu. It applies to thin products, that is to say, to sheets, strips and tubes at most 15 mm thick and to long products such as bars, sections etc., or products of any shape, to those having an equivalent thickness at most equal to 15 mm, i.e., the equivalent thickness is double the ratio of the volume to the sum of the lateral surfaces.
Conventional treatment for hardening such alloys comprise the following stages, in order:
1--solution heat treatment; PA1 2--quenching; and, PA1 3--tempering, or ageing. PA1 (3a) pre-tempering in the zone from 100.degree. to 150.degree. C. for a period lasting from 5 minutes to 24 hours; PA1 (3b) intermediate tempering at a higher temperature; and, PA1 (3c) final tempering for 2 to 48 hours at between 100.degree. and 160.degree. C. PA1 T is the total period, in seconds, of this stage, starting from the moment when the temperature of the product reaches 190.degree. C. for the first time, in the sense of the rise in the temperature; PA1 .theta.(t) is the temperature in .degree.K above 463.degree. K., that is 190.degree. C., of the coldest point of the product; PA1 t is the time in seconds; and PA1 the temperature .theta.(t) always being less than 523.degree. K., 250.degree. C., and, preferably, below 508.degree. K., 235.degree. C. PA1 34 log To=225-.theta., .theta. being in .degree.C.; and PA1 log=decimal logarithm.
optionally with cold plastic deformation of 1 to 5% between the second the third stages for stress-relieving the products in the crude state of quenching.
This plastic deformation is generally obtained by controlled stretching in the case of flat products, TXX51 state.
The tempering treatment leading to the highest mechanical tensile characteristics generally involves increasing the temperature to below 140.degree. C., keeping the temperature isothermal and cooling. This state is known as a T6, or T651 state, leads to a very poor resistance to stress corrosion in the short transverse direction and to exfoliation corrosion.
In order to overcome this disadvantage, a treatment involving a first isothermal step at a temperature below 140.degree. C. followed by a second isothermal step at a temperature above 150.degree. C. and cooling is generally practiced, each step being preceded by a slow rise in temperature. It is intended to impart to the alloys a high resistance to stress corrosion in the short transverse direction, but this is associated with a very substantial reduction in their mechanical characteristics with respect to the T6, or T651 state.
This state is known by those skilled in the art as the T73, or T7351 state.
There is finally a tempering treatment intended to impart to rolled products mechanical tensile characteristics and resistance to stress corrosion which fall between those of the T6, or T651 and T73, or T7351 states, with a high resistance to exfoliation corrosion. This tempering treatment is similar to the T73, or T7351 state tempering treatment, but the periods of treatment are generally shorter in it. This state is known as a T76, or T7651 state, by those skilled in the art, and is applied to sheets of thin or medium thickness.
The resistance to stress corrosion is generally evaluated on samples, which have been cut out in the short transverse direction, by means of alternate immersion and emersion tests, e.g., 10 minutes-50 minutes, in a reagent containing 3.5% NaCl in accordance with American Standard ASTM G44-75, Standard Recommended Practice for Alternate Immersion Stress Corrosion Testing in 3.5% Sodium Chloride Solution.
The resistance to exfoliating corrosion is evaluated by the Exco test in accordance with American Standard ASTM G34-72, Standard Method of Test for Exfoliation Corrosion Susceptibility in 7XXX Series Copper-Containing Aluminum Alloys.